Ethylene vinyl alcohol (EVOH) copolymers are recognized to have considerable value in packaging films, and especially those intended for use with foods and other products that require protection against oxygen. This is, of course, because of the excellent oxygen barrier properties of EVOH copolymer films, which are exhibited even in sections as thin as about 0.1 mil or less. Despite this, the EVOH resins suffer from a number of drawbacks.
In particular, films made from EVOH tend to lack toughness and to be relatively brittle, particularly in the thin sections in which they are most economically used. Moreover, of the films with which they are desirably and most practically combined, the EVOH copolymers exhibit good adhesion only to those made of nylon, thus necessitating the utilization of supplemental adhesives when other resins are to be joined thereto. The EVOH copolymers are also quite expensive.
On the other hand, it is also well recognized that films of high molecular weight, relatively high melting point polyamides exhibit outstanding levels of toughness and tensile strength, while at the same time offering good adhesion to a variety of resinous substrates or coatings; however, nylon is not an effective oxygen barrier polymer, for certain applications. Consequently, separate layers of EVOH copolymers and polyamides are presently employed in composite film structures, so as to obtain a desirable combination of the properties that are afforded by each.
Although it is entirely feasible to produce composite films of the foregoing nature by coextrusion or by extrusion coating, it is found to be rather difficult to achieve the measure of control necessary to produce uniform layers of the components, optimal adhesion, and a desirable balance of properties. Moreover, property variations are not readily made, as is desirable in many instances to tailor the film to use for specific applications.
It is believed that attempts have been made in the past to produce blends of polyamide resins (as well as other polymers) with ethylene vinyl alcohol copolymers, in an endeavor to provide extended compositions from which single-layer films exhibiting desirable properties might be produced. However, as far as is known such attempts have been unsuccessful, due primarily to compatibility problems and the difficulty of extruding the blended resins, while preserving the desirable properties of the components. In particular regard to the film-forming polyamide resins, those which exhibit the desired properties are processable only at temperatures that are considerably above the processing temperatures of the EVOH resins. Consequently, it has not heretofore been possible to extrude a blend of the two ingredients without intolerable degradation (essentially gel formation) of the copolymer. Moreover, it would be anticipated that, in any such blend, the excellent oxygen barrier characteristics of the EVOH copolymer would be significantly compromised, due to the presence of the polyamide resin, and that, conversely, the inherent physical or structural properties of the polyamide would be diminished in direct proportion to the amount of EVOH copolymer included.
Accordingly, it is a primary object of the present invention to provide a novel blend of a film-forming polyamide resin and an EVOH copolymer, which blend is processable at temperatures below those at which any substantial degradation of the EVOH copolymer will be encountered, to produce a film having a desirable balance of oxygen barrier and structural properties.
It is also an object of the invention to provide such a blend which is adapted to be coextruded with a second resin, to produce a composite film in which adhesion between the layers is at a desirable level.
Another object of the invention is to provide a novel process by which films having relatively high levels of strength and toughness, and excellent oxygen barrier characteristics, can be produced with relative facility and good economy, which process may entail the coextrusion with a second, distinct resin.
A further object of the invention is to provide novel products produced by the methods of the invention.